1. Field of the Invention
The present invention relates to an Ni-base superalloy (i.e., super heat resisting alloy) which is suitable for use as the material for disks or the like of a gas turbine, which can be hot worked and which has a high strength comparable to that of powder metallurgy alloy, and to a method for producing the same.
2. Description of the Prior Art
Current trends for greater output and higher efficiency of gas turbines naturally require that heat resisting parts of gas turbines operate under severer conditions. In case of disks of gas sturbines, an increasing demand exists for improvement in the mechanical strength of the material for disks rather than for a rise in the maximum withstandable temperature of the disks. Thus, the following two kinds of approaches have been made to increase the performance of turbine disk material.
(1) Development of novel alloy having high .gamma.' phase content by powder metallurgy process.
(2) Improvement in the strength of existing ingot alloy by thermomechanical treatment.
As an example of Ni-base superalloy according to the approach (1), such a high strength alloy having .gamma.' phase content of about 50 vol.% as known under the name of RENE' 95 (RENE' being a trademark) or IN 100 (IN being a trademark) has been put to practical use in commercial base.
The RENE' 95 is an alloy which is disclosed in Japanese Patent Examined Publication No. 46-22333. Initially, it was attempted to fabricate this alloy by the conventional ingot making and subsequent hot working process. This attempt, however, was unsuccessful because of difficulty in fabricating this alloy from the ingot material due to containing a large amount of .gamma.' phase, so that this alloy is fabricated only by powder metallurgy process at present. On the other hand, the IN 100 has been developed as a cast alloy from the beginning, so that no attempt has been made to commercially fabricate this alloy by the ingot making and hot working process.
Further, Japanese Patent Unexamined Publication No. 63-114933 discloses an alloy which exhibits superior properties as a material for gas turbine disks. This alloy, however, also is a high .GAMMA.' alloy containing about 45% of .gamma.' phase and, therefore, cannot be fabricated by the conventional ingot making and hot working process.
Thus, an alloy having high .gamma.' phase content becomes impossible to be hot worked and, hence, is obliged to adopt powder metallurgy process. The powder metallurgy process, however, employs a number of steps so that the price is raised uneconomically. In addition, the powder metallurgy process tends to allow the product to contain oxides, impairing the reliability of the product.
According to the approach (2) mentioned above, thermomechanical treatment, which is a combination of a hot working and a heat treatment, is effected on an Ni-base superalloy such as WASPALLOY (WASPALLOY being a trademark) or INCONEL 718 (INCONEL being a trademark), in order to achieve desired performance. Alloys obtained by such thermomechanical treatment exhibit mechanical properties which are excellent in comparison with conventional ingot alloys but are still inferior in comparison with those exhibited by supperalloys produced by the powder metallurgy process of the aforesaid approach (1).
Further, Japanese Patent Unexamined Publication No. 63-145737 discloses an alloy which is said to be a high-strength ingot alloy having .gamma.' phase content of 45 vol.% and exhibiting superior hot workability. However, it is very difficult to hot work this alloy and an extremely high degree of forging technique is required due to the .gamma.' phase content which is much higher than that of existing ingot alloy.
Considering merits and demerits of the aforementioned approaches (1) and (2) for increasing the performance of disk material, it is highly desirable to develop an alloy which can be produced by a process making use of existing production equipment, e.g., a process having the steps of conventional ingot making and hot working, and which has properties well comparable to those of alloys produced by powder metallurgy process, because such an alloy will enable inexpensive production of large-sized parts having high reliability.